Typically, gas turbine engines include a plurality of injectors for injecting fuel into a combustor to mix with air upstream of a flame zone. The fuel injectors of conventional turbine engines may be arranged in one of at least three different schemes. Fuel injectors may be positioned in a lean premix flame system in which fuel is injected in the air stream far enough upstream of the location at which the fuel/air mixture is ignited that the air and fuel are completely mixed upon burning in the flame zone. Fuel injectors may also be configured in a diffusion flame system such that fuel and air are mixed and burned simultaneously. In yet another configuration, often referred to as a partially premixed system, fuel injectors may inject fuel upstream of the flame zone a sufficient distance that some of the air is mixed with the fuel. Partially premixed systems are combinations of a lean premix flame system and a diffusion flame system.
In addition, axial stage fuel nozzles have been positioned within secondary combustion zones downstream from the main combustion zones. The axial stage fuel nozzles enable high temperature operation with low NOx and reduced combustor dynamics. The axial stage fuel nozzles can emit fuel or a mixture of fuel and a diluent (typically steam) downstream of the main combustion zones. The axial stage fuel nozzles will typically fail if the fuel or diluent steam flow is shut off. While steam diluent is effective to use as a coolant, it is also expensive. Thus, a more cost efficient cooling system is needed.